feat(v5 slice 6): export clamps + cable mid-vertices to mxdrw

Excalidraw scene now mirrors the v5 routing model:

- Clamps export as 12×12 grey rounded squares (BackgroundColor=#888888,
  StrokeColor=#555555, Roundness type 3). Distinct from the red IO
  marker diamonds so wall outlets vs. routing anchors stay readable.
  Frame_id propagates into the element's FrameID per the existing
  pattern.
- Cable arrows include clamp positions as mid-vertices in the
  `points` array. Pre-grouped + sort.Slice-sorted by ord; each
  mid-vertex is added as an (x-fromAnchor.x, y-fromAnchor.y) offset.
  startBinding / endBinding still point at the from / to endpoint
  excalidraw_ids; mid-vertices are unbound (Excalidraw doesn't have
  per-vertex binding).
- IDAssignment grows a Clamps map; PersistExcalidrawIDs accepts it
  and updates clamps.excalidraw_id on first export so re-exports
  reuse the same element ids (collab cursors / undo history survive).
- Bundle-stripe overlay is **viewer-only** — Excalidraw can't
  represent gradient strokes losslessly, so we export individual
  cable arrows and let the in-app viewer derive the bundle viz.

Tests:
- TestBuildScene_ClampsRenderAsRectangles — 2 clamps → 2 rectangle
  elements + 2 ids in IDAssignment.Clamps.
- TestBuildScene_ArrowPointsIncludeClamps — cable with 1 clamp →
  arrow.Points has 3 entries; middle vertex equals the clamp's
  position relative to fromAnchor.

This closes the v5 slice plan (§11.10). Six slices, one branch,
one redeploy below.

internal/db/excalidraw_ids.go

@@ -12,7 +12,7 @@ import (
 // Caller passes one map per kind; keys are the in-project row ids,
 // values are the 21-char Excalidraw element ids the exporter minted.
 func (s *Store) PersistExcalidrawIDs(projectID int64,
-	frames, devices, ports, ios, cables map[int64]string,
+	frames, devices, ports, ios, cables, clamps map[int64]string,
 ) error {
 	tx, err := s.db.Begin()
 	if err != nil {
@@ -35,6 +35,9 @@ func (s *Store) PersistExcalidrawIDs(projectID int64,
 	if err := updateExIDs(tx, "cables", projectID, cables); err != nil {
 		return err
 	}
+	if err := updateExIDs(tx, "clamps", projectID, clamps); err != nil {
+		return err
+	}
 	return tx.Commit()
 }
 

internal/exporter/exporter.go

@@ -11,6 +11,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"math/big"
+	"sort"
 
 	"mgit.msbls.de/m/mcables/internal/db"
 )
@@ -114,6 +115,7 @@ type IDAssignment struct {
 	Ports     map[int64]string `json:"ports"`
 	IOMarkers map[int64]string `json:"io_markers"`
 	Cables    map[int64]string `json:"cables"`
+	Clamps    map[int64]string `json:"clamps"`
 }
 
 // BuildScene transforms a project snapshot into an Excalidraw Scene +
@@ -132,6 +134,7 @@ func BuildScene(snap *db.Snapshot, nowMilli int64, genID func() string) (*Scene,
 		Ports:     map[int64]string{},
 		IOMarkers: map[int64]string{},
 		Cables:    map[int64]string{},
+		Clamps:    map[int64]string{},
 	}
 	// idFor: reuse the existing excalidraw_id if present, else mint one.
 	idFor := func(existing *string) string {
@@ -381,6 +384,58 @@ func BuildScene(snap *db.Snapshot, nowMilli int64, genID func() string) (*Scene,
 		})
 	}
 
+	// Clamps — small grey rounded squares (v5 §11.7). Distinct from the
+	// red IO marker diamonds so m can tell routing anchors from wall
+	// outlets at a glance.
+	const clampSize = 12.0
+	for _, cl := range snap.Clamps {
+		elID := idFor(cl.ExcalidrawID)
+		a.Clamps[cl.ID] = elID
+		var frameRef *string
+		if cl.FrameID != nil {
+			if v, ok := frameElID[*cl.FrameID]; ok {
+				frameRef = &v
+			}
+		}
+		els = append(els, Element{
+			ID:              elID,
+			Type:            "rectangle",
+			X:               cl.X - clampSize/2,
+			Y:               cl.Y - clampSize/2,
+			Width:           clampSize,
+			Height:          clampSize,
+			StrokeColor:     "#555555",
+			BackgroundColor: "#888888",
+			FillStyle:       "solid",
+			StrokeWidth:     1,
+			StrokeStyle:     "solid",
+			Roughness:       0,
+			Opacity:         100,
+			GroupIDs:        []string{},
+			FrameID:         frameRef,
+			Roundness:       &Roundness{Type: 3},
+			Seed:            randInt(),
+			Version:         1,
+			VersionNonce:    randInt(),
+			Updated:         nowMilli,
+		})
+	}
+
+	// Pre-group cable_clamps by cable for the arrow mid-points pass.
+	clampsByCable := map[int64][]db.CableClamp{}
+	for _, cc := range snap.CableClamps {
+		clampsByCable[cc.CableID] = append(clampsByCable[cc.CableID], cc)
+	}
+	for _, arr := range clampsByCable {
+		// Already sorted by ListCableClamps (ORDER BY cable_id, ord),
+		// but defend against unsorted inputs.
+		sort.Slice(arr, func(i, j int) bool { return arr[i].Ord < arr[j].Ord })
+	}
+	clampPos := map[int64][2]float64{}
+	for _, cl := range snap.Clamps {
+		clampPos[cl.ID] = [2]float64{cl.X, cl.Y}
+	}
+
 	// Cables — arrows with startBinding/endBinding to the port / device /
 	// IO marker excalidraw_ids. Endpoint anchors (the visible "from" /
 	// "to" points) come from the same anchor logic the canvas uses.
@@ -403,6 +458,18 @@ func BuildScene(snap *db.Snapshot, nowMilli int64, genID func() string) (*Scene,
 		}
 		startArr := ""
 		endArr := "arrow"
+		// Excalidraw arrow `points` is relative to (X, Y). We anchor at
+		// the from-point, so vertex 0 is always (0, 0). Mid-vertices
+		// (clamps) and the final to-vertex are offsets from there.
+		pts := [][2]float64{{0, 0}}
+		for _, cc := range clampsByCable[c.ID] {
+			pos, ok := clampPos[cc.ClampID]
+			if !ok {
+				continue
+			}
+			pts = append(pts, [2]float64{pos[0] - fromAnchor[0], pos[1] - fromAnchor[1]})
+		}
+		pts = append(pts, [2]float64{toAnchor[0] - fromAnchor[0], toAnchor[1] - fromAnchor[1]})
 		els = append(els, Element{
 			ID:              elID,
 			Type:            "arrow",
@@ -422,7 +489,7 @@ func BuildScene(snap *db.Snapshot, nowMilli int64, genID func() string) (*Scene,
 			Version:         1,
 			VersionNonce:    randInt(),
 			Updated:         nowMilli,
-			Points:          [][2]float64{{0, 0}, {toAnchor[0] - fromAnchor[0], toAnchor[1] - fromAnchor[1]}},
+			Points:          pts,
 			StartArrowhead:  &startArr,
 			EndArrowhead:    &endArr,
 			StartBinding:    bindingPtr(fromRef),

internal/exporter/exporter_test.go

@@ -137,6 +137,66 @@ func TestBuildScene_BundlesIgnored(t *testing.T) {
 	}
 }
 
+func TestBuildScene_ClampsRenderAsRectangles(t *testing.T) {
+	snap := sampleSnapshot()
+	snap.Clamps = []db.Clamp{
+		{ID: 1, ProjectID: 1, X: 500, Y: 300},
+		{ID: 2, ProjectID: 1, X: 550, Y: 320},
+	}
+	scene, ids := BuildScene(snap, 1700000000000, newSeq())
+	if len(ids.Clamps) != 2 {
+		t.Errorf("clamp ids = %d, want 2", len(ids.Clamps))
+	}
+	clampElIDs := map[string]bool{}
+	for _, id := range ids.Clamps {
+		clampElIDs[id] = true
+	}
+	got := 0
+	for _, e := range scene.Elements {
+		if clampElIDs[e.ID] && e.Type == "rectangle" {
+			got++
+		}
+	}
+	if got != 2 {
+		t.Errorf("clamp rectangle elements = %d, want 2", got)
+	}
+}
+
+func TestBuildScene_ArrowPointsIncludeClamps(t *testing.T) {
+	snap := sampleSnapshot()
+	snap.Clamps = []db.Clamp{
+		{ID: 10, ProjectID: 1, X: 350, Y: 250},
+	}
+	snap.CableClamps = []db.CableClamp{
+		{CableID: 1000, ClampID: 10, Ord: 1},
+	}
+	scene, _ := BuildScene(snap, 1700000000000, newSeq())
+	var arrow *Element
+	for i := range scene.Elements {
+		if scene.Elements[i].Type == "arrow" {
+			arrow = &scene.Elements[i]
+			break
+		}
+	}
+	if arrow == nil {
+		t.Fatal("no arrow in scene")
+	}
+	if len(arrow.Points) != 3 {
+		t.Errorf("arrow points = %d, want 3 (from + clamp + to): %+v", len(arrow.Points), arrow.Points)
+	}
+	// First point is always (0, 0) by convention; middle point should
+	// equal the clamp's position relative to the arrow's anchor.
+	if arrow.Points[0][0] != 0 || arrow.Points[0][1] != 0 {
+		t.Errorf("first point = %v, want [0,0]", arrow.Points[0])
+	}
+	// Middle vertex = clamp.x - fromAnchor.x, clamp.y - fromAnchor.y.
+	// fromAnchor for port 100 = (200 + 50, 200 + 35) = (250, 235).
+	wantX, wantY := 350.0-250.0, 250.0-235.0
+	if arrow.Points[1][0] != wantX || arrow.Points[1][1] != wantY {
+		t.Errorf("mid point = %v, want [%v, %v]", arrow.Points[1], wantX, wantY)
+	}
+}
+
 func TestMarshalScene_IsJSON(t *testing.T) {
 	snap := sampleSnapshot()
 	scene, _ := BuildScene(snap, 1700000000000, newSeq())

internal/server/export.go

@@ -51,7 +51,7 @@ func (h *handlers) syncExport(w http.ResponseWriter, r *http.Request) {
 	// Persist the freshly-assigned ids so the next export reuses them.
 	// We pass in the full maps; PersistExcalidrawIDs is idempotent (it
 	// only updates rows whose excalidraw_id is still NULL).
-	if err := h.store.PersistExcalidrawIDs(pid, ids.Frames, ids.Devices, ids.Ports, ids.IOMarkers, ids.Cables); err != nil {
+	if err := h.store.PersistExcalidrawIDs(pid, ids.Frames, ids.Devices, ids.Ports, ids.IOMarkers, ids.Cables, ids.Clamps); err != nil {
 		writeError(w, fmt.Errorf("persist excalidraw_ids: %w", err), nil)
 		return
 	}